There are several reasons why SageMath cannot use Debian's maxima package at the moment
and this second package maxima-sage is needed:
1. Version mismatch
SageMath 7.4 uses Maxima 5.35.1, while the maxima package in Debian is constantly updated to follow
new upstream versions. Currently the version of maxima in Debian is 5.38.1. It is not trivial
to update SageMath to a new Maxima version, as this ticket for the Maxima 5.38.1 update shows:
https://trac.sagemath.org/ticket/18920
2. Lisp compiler
The maxima package in Debian uses only GCL as common lisp compiler, while SageMath uses ECL. SageMath
cannot use the GCL version, since the preferred interface to Maxima is via an ECL fasl library.
The possibility of adding an ECL version of Maxima to the maxima package was discussed in
https://bugs.debian.org/779804
This maxima-sage package is coinstallable with the maxima packages in Debian.